Absorbent structures comprising a substrate, and frequently a capillary absorbent substrate are known in the art. As used herein, a "capillary" absorbent structure absorbs liquids, such as water, by capillary attraction of the liquids due to the thermodynamic force of attraction between a liquid and the solid surface of the capillary medium.
Frequently an osmotic absorbent is applied to or otherwise disposed on the substrate. As used herein, an "osmotic" absorbent structure absorbs liquids deposited thereon by equalization of differential partial fluid pressure in the absence of ion exchange, forming a gelatinous substance which imbibes the liquids. As used herein, an "absorbent structure" refers to materials which, in combination, absorb liquids by both osmotic and capillary absorptions.
The osmotic absorbent may be made from monomers selected from the group consisting of acrylic acid, starch grafted acrylate co-polymers, etc. Such osmotic absorbent materials are commonly used as absorbent gelling materials or superabsorbers in disposable absorbent articles such as diapers and sanitary napkins. The osmotic absorbent may be applied to the substrate in the form of a liquid precursor, to be later cured into an osmotic absorbent.
The capillary absorbent may be provided in the form of a substrate, for the osmotic absorbent to be later applied thereupon. Typically the capillary absorbent substrate is a generally planar, almost two-dimensional material, such as paper, nonwoven fabric, woven fabric, or even formed film.
Generally, the osmotic absorbent material may be applied to the capillary absorbent substrate as a fluid precursor, such as a liquid monomer, then crosslinked to form an absorbent polymeric material. Usually, the liquid precursor is applied to the capillary absorbent substrate in a fluid form and typically comprises some form of acrylic acid and acrylate salts.
Typically, the liquid precursor is applied to the absorbent substrate by spraying, impregnation, etc. to provide a uniform coating thereon. Other teachings in the art suggest discontinuous applications of the liquid precursor to the substrate through brushing, roller coating, etc. Once the liquid precursor is applied to the capillary absorbent substrate, the liquid precursor may be crosslinked through elevated temperature, irradiation, etc.
Examples of such attempts in the art include U.S. Pat. Nos. 4,008,353 issued Feb. 15, 1977 to Gross et al.; 4,061,846 issued Dec. 6, 1977 to Gross et al.; 4,071,650 issued Jan. 31, 1978 to Gross; 4,835,020 issued May 30, 1989 to Itoh et al.; 4,842,927 issued Jun. 27, 1989 to Itoh et al.; 4,865,886 issued Sep. 12, 1989 to Itoh et al; 4,892,754 issued Jan. 9, 1990 to Itoh et al.; 5,079,034 issued Nov. 21, 1988 to Miyake et al. and Great Britain Patent 1,452,325 published October, 1976 in the name of Triopolis.
However, these attempts in the art suffer from serious drawbacks. As is all too well known in the art, when an osmotic absorbent imbibes liquids, the osmotic absorbent swells in volume. If such swelling occurs too rapidly, the increase in volume of the osmotic absorbent which has imbibed liquids may prevent later liquid insults from reaching portions of the osmotic absorbent which are still able to absorb liquids. This phenomenon, known as gel blocking, may prevent further absorption of liquids. Gel blocking often prevents the absorbent structure from utilizing its total capacity. If an absorbent structure which encounters gel blocking is used in a disposable absorbent article, such as a diaper or sanitary napkin, and liquid insults occur after the gel blocking, such insults may not be absorbed and leakage may result.
Clearly from this standpoint, a uniform coating of the liquid precursor material on the capillary substrate can be very undesirable. However, a high surface area to mass ratio of the osmotic absorbent generally increases the rate of absorbency. Therefore, to minimize gel blocking a thin nonuniform coating of the osmotic absorbent may be applied to the capillary substrate as is known in the art.
Typically, the capillary substrate (and the machinery and the papermaking clothing used to manufacture the capillary substrate) are selected based upon the needs of the consumer. The processes used to make the capillary substrate are often custom designed to meet the tradeoffs inherent in balancing the different properties (e.g., tensile strength, softness, absorbency) which affect the consumers' likes and dislikes, and ultimately the sales of the absorbent structure incorporating the capillary substrate. However, difficulties can arise in the prior art methods of applying the liquid precursor to the capillary substrate.
For example, it is difficult to spray the liquid precursor onto the substrate in a precise pattern. Printing the osmotic absorbent onto the substrate may result in a pattern having greater definition and precision than obtainable by spraying, but requires a printing roll having raised protuberances or gravure cells. Printing rolls having raised protuberances and gravure plates limit the pattern of the applied osmotic absorbent to that pattern corresponding to the protuberances of the printing roll or the gravure plates, regardless of which pattern may be desirable for a particular capillary substrate.
This problem may be overcome by providing a plethora of printing rolls and gravure plates, one for each desired pattern. However, such provision increases the expense of the apparatus to a point where it may not be economically feasible to provide a printing roll or a gravure plate for each desired pattern if only a short production run is desired.
Furthermore, the substrates disclosed in the prior art often exacerbate the gel blocking problem. The common uniform basis weight and uniform density capillary substrates provide equal capillary absorption in the X-Y plane. Insults of liquid deposited onto such a capillary substrate wick throughout all regions of the capillary substrate. Such wicking may transport the liquids into a region which is already gel blocked. Alternatively, the capillary absorbent may not compete sufficiently with the osmotic absorbent material to fully utilize the entire capacity of the absorbent structure.
Yet other problems encountered in the prior art include migration of the liquid precursor after it is applied to the capillary substrate. Such migration occurs in the X-Y plane. X-Y migration diminishes the differences in the pattern between the areas of the capillary substrate to which the liquid precursor was and was not applied.
Migration of the liquid precursor also occurs in the Z-direction, normal to the plane of the capillary substrate. Z-direction migration causes the liquid precursor to penetrate the thickness of the capillary substrate, to a uniform distribution between both faces of the capillary substrate. This uniform Z-direction distribution may limit the free swelling of the osmotic absorbent resulting from the liquid precursor, limiting its ability to absorb further liquid insults.
Unfortunately, physical constraints imposed by the capillary substrate itself which surrounds the osmotic absorbent distributed in the Z-direction, limit its ability to swell in the presence of liquid insults. Such limitations are directly proportional to the density of the capillary substrate into which the osmotic absorbent is disposed and are inversely proportional to the quantity and extent of the osmotic absorbent disposed out of the plane of the capillary substrate in the Z-direction.
Accordingly, it is an object of this invention to provide an absorbent structure which minimizes gel blocking by providing a pattern of an osmotic absorbent on a capillary substrate. Further, it is an object of this invention to provide an absorbent structure which allows swelling of the osmotic absorbent to occur without constraints being imposed by the capillary substrate. Finally, it is an object of this invention to provide an absorbent structure having a relatively high absorbency rate for a given gel strength by providing a favorable surface area to mass ratio.